Termination for gas cooled cryogenic power cables

ABSTRACT

A cable termination comprising an upper cryostat chamber containing a liquid cryogen and a lower cryostat chamber containing a gaseous cryogen to maintain dielectric integrity and thermal management of an electric connection. A gaseous cryogen recirculation system may cause the gaseous cryogen to flow through the lower cryostat chamber and a power cable enclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This nonprovisional application is a continuation-in-part of and claims priority to U.S. nonprovisional application Ser. No. 13/890,816, entitled “Cable Termination for High Voltage Power Cables Cooled by a Gaseous Cryogen,” filed May 9, 2013, which claims priority to provisional application No. 61/645,304, entitled “Cable Termination for High Voltage Power Cables Cooled by a Gaseous Cryogen,” filed May 10, 2012, and provisional application No. 61/697,567, entitled “Cryogenic Heat Sink for Helium Gas Cooled Superconducting Power Devices,” filed Sep. 6, 2012, all of which are incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No. N00014-02-1-0623 awarded by the Office of Naval Research. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, generally, to cryogenic power cables or superconducting power cables. More particularly, it relates to terminations used in medium voltage (<˜63 kV) power cables (e.g., superconducting cables) and methods of maintaining temperatures and integrities thereof.

2. Description of the Prior Art

A high voltage cable termination has the purpose to allow an electric connection of a cable with connectors, conductors, or bus bars. The termination must allow dielectric integrity and thermal management of a cable at any point of operation, in particular for any permissible load current and ambient temperature, when cooled by a gaseous cryogen. The termination must also maintain the conductor below a critical temperature if superconducting materials are used.

Dielectric integrity is challenging in systems with gaseous cryogen used as both coolant and electric insulation. Gaseous media have a low dielectric strength compared to liquid media. However, gaseous media are considerably better at cryogenic temperature and elevated pressure compared to standard temperature and pressure (STP) conditions. It is of paramount importance for the dielectric integrity of the cable termination that any gaseous cryogen is at low temperature and high pressure, particularly on the solid insulator surfaces and at the location of the superconductor.

Careful thermal management is critical in order to keep the conductor in the cable cold (i.e., minimize heat influx to the cable) and at the same time minimize the required power to cool the termination and the cable. Thermal management is interconnected with dielectric integrity since the temperature of the gaseous cryogen is critical for dielectric integrity.

Power cables made of high temperature superconductors (HTS) have been of interest because of the availability of the materials in long length tapes [1, 2, 3]. Many prototype demonstrations of HTS power cables have been successfully completed. A majority of the demonstrations were AC power cables cooled with liquid nitrogen.

However, long distance superconducting direct current (DC) power transmission cables were found to be advantageous [4, 5, 6, 7]. One of the attractions of DC transmission systems is their suitability for smart grid concepts where power can easily be transmitted from large scale renewable energy sources, such as wind farms, photovoltaic farms, and ocean wave power generators, as well as interconnection between independently-controlled power grids and long distance power transmission. Another advantage of superconducting DC transmission and distribution systems is the absence of AC losses in the cables that add to the cryogenic capacity required to operate the cables. There have been a limited number of demonstration studies of superconducting DC power cables cooled with liquid nitrogen.

More recently, there has been research exploring the possibility of using cryogenic helium gas circulation for cooling HTS power cables [8, 9]. The primary advantage of helium, as compared to liquid nitrogen, is the possibility of lower operating temperatures that allow the cable designs to take advantage of the significantly higher critical currents of HTS tapes, thus allowing the cables to be smaller in size and weight. Reduced size and weight are particularly attractive for naval and aviation applications [10, 11, 12, 13].

The United States Navy, National Aeronautics Space Administration and Air Force Research Laboratory have been investigating all-electric ships and all-electric airplanes based on superconducting cables, motors, and generators [13]. Lower operating temperatures are also essential for cables based on MgB₂ superconductors whose superconducting transition temperature is around 39 K and typical cable operating temperatures are between 10 K and 20 K [9]. Circulation of gaseous or liquid neon and hydrogen are potential alternatives, but for cost and safety reasons, gaseous helium circulation has been the choice for ongoing research of superconducting cables [10, 11, 12].

Superconducting DC cables typically do not generate any heat, though different cryogens have different heat capacities and thermal conductivities, thus having a substantial impact on the thermal aspects of the cable. Superconducting degaussing cables cooled with gaseous helium circulation have successfully been demonstrated [10, 11]. A power transmission or distribution cable system consists of the terminations that transfer several kiloamperes of current from the network at ambient temperature to the superconducting cable at cryogenic temperatures. The heat from the ambient through the current leads and the termination tanks, along with the Joule heating (the generation of heat by passing electric current through a conductor) at the resistive joints between the external current leads and the internal terminals (e.g., aluminum, copper, other metal alloys contributing to Joule heating) of the superconducting cable, is significant.

This heat load at the terminations is one of the most challenging design aspects of helium gas cooled superconducting cables. Another design challenge for helium gas cooled superconducting cables is the substantially lower dielectric strength of helium gas [14, 15, 16]. The challenge is exasperated at the terminations because the warmer interfaces reduce the local gas density, thus making the dielectric strength weaker [16].

Accordingly, what is needed is an apparatus and method for more efficiently maintaining thermal and dielectric integrity in cable terminations, in particular those of gas cooled superconducting cables. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the art could be advanced.

All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.

The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.

SUMMARY OF THE INVENTION

The long-standing but heretofore unfulfilled need for an improved, more effective and more efficient cable termination is now met by a new, useful and nonobvious invention.

Various embodiments may comprise an apparatus for maintaining dielectric integrity and thermal management of an electric connection. A power cable termination may comprise an upper cryostat chamber and a lower cryostat chamber. The upper cryostat chamber may have an interior cavity at least partially filled with a liquid cryogen. The lower cryostat chamber may have an interior cavity at least partially filled with a gaseous cryogen. A conductor may extend into the interior cavity of the upper cryostat chamber, and a power cable may extend into the interior cavity of the lower cryostat chamber. The power cable may be electrically coupled to the conductor.

Other embodiments may comprise an apparatus for maintaining dielectric integrity and thermal management of an electric connection. A power cable termination may comprise an upper cryostat chamber and a lower cryostat chamber. The upper cryostat chamber may have an interior cavity at least partially filled with a liquid cryogen. The lower cryostat chamber may have an interior cavity at least partially filled with a gaseous cryogen. An upper bushing may extend into the upper cryostat chamber, and a lower bushing may extend into the lower cryostat chamber. An upper bushing conductor may pass through the upper bushing and into the interior cavity of the upper cryostat chamber. A portion of the upper bushing conductor may be positioned outside the power cable termination, and a portion of the upper bushing conductor may be immersed in the liquid cryogen. A lower bushing conductor may pass through the lower bushing and into the lower cryostat chamber such that a portion of the lower bushing conductor is positioned within the gaseous cryogen and a portion of the lower bushing conductor is immersed in the liquid cryogen. A power cable may extend into the interior cavity of the lower cryostat chamber and may be electrically coupled to the lower bushing conductor.

Still other embodiments may comprise a system for maintaining dielectric integrity and thermal management of an electric connection. A cable enclosure may contain a power cable and may have an inlet end and an outlet end. A first cable termination may be coupled to the cable enclosure inlet end and a second cable termination coupled to the cable enclosure outlet end. Each of the first and second cable terminations may comprise an upper cryostat chamber containing a liquid cryogen and a lower cryostat chamber containing a gaseous cryogen. Each lower cryostat chamber may further comprise an inlet and an outlet. A gaseous recirculation system may be configured to cause the gaseous cryogen to flow through the first cable termination, through the cable enclosure, and through the second cable termination.

These and other important objects, advantages, and features of the invention will become clear as this disclosure proceeds.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the disclosure set forth hereinafter and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed disclosure, taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a cable termination system according to various embodiments.

FIG. 2 is a front sectional view of a cable termination according to various embodiments.

FIG. 3 is a graph of temperature measured at inlet and outlet of terminations without liquid cryogen in the upper cryostat chamber.

FIG. 4 is a graph of temperature rise in 1 m and 30 m cable enclosures without liquid cryogen in the upper cryostat chamber of the termination at various gas cryogen mass flow rates.

FIG. 5 is a graph of heat leak components without liquid cryogen in the upper cryostat chamber for 1 m and 30 m cable enclosures at various gas cryogen mass flow rates.

FIG. 6 is a graph of heat leak components with liquid cryogen in the upper cryostat chamber for 1 m and 30 m cable enclosures at various gas cryogen mass flow rates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part thereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.

Dielectric properties of gases are only acceptable for use as an insulator at lowest temperatures. Similarly, superconducting cables only allow useful current densities at lowest temperatures. A cost effective solution to achieve low gas temperatures at a surface of a feed through and a conductor coupled to the superconducting cable comprises using a liquid cryogen to cool the feed through. However, the prior art has not achieved this solution effectively.

The present application describes a novel cable termination within a cooled superconducting power cable system combining gaseous and liquid cryogens. The introduction of a liquid cryogen may ensure that dielectric surfaces exposed to the gaseous cryogen as well as parts on high voltage potential are held at cryogenic temperatures within the cooled superconducting power cable system. The introduction of capacitive grading minimizes an electric field on the surface of a bushing in the vapor phase of the cryogen used in a liquid cryogen compartment. The change of cross-section of the conductor along its axis enables thermal optimization for reduction in the loss of liquid cryogen.

Certain embodiments of the current invention describe an apparatus for maintaining both dielectric and thermal integrity by introducing a liquid cryogen in a superconducting cable system. The liquid cryogen has dielectric strength substantially higher than the gaseous cryogen used in the cable, in part due to the higher density of the liquid cryogen. At the same time, the liquid cryogen features increase heat conduction, which facilitates keeping all parts at cryogenic temperature that are at high voltage and in contact with the gaseous cryogen. The liquid cryogen also reduces heat influx into the superconducting materials. For example, a suitable combination of cryogens may be liquid nitrogen and high-pressure gaseous helium, the former being the liquid cryogen in the cable system and the latter being the gaseous cryogen in the cable system.

FIG. 1 illustrates a schematic diagram of a cooled superconducting power cable system 100 according to various embodiments. A cable enclosure 130 spans a distance between a first cable termination 105 and a second cable termination 110. A superconducting cable 280 (see FIG. 2) may be positioned within the cable enclosure 130. A first end of the superconducting cable 280 may be coupled to the first cable termination 105, and a second end of the superconducting cable 280 may be coupled to the second cable termination 110.

Each of the first and second cable terminations 105, 110 may comprise an upper cryostat chamber 120 and a lower cryostat chamber 125. In various embodiments, the upper cryostat chamber 120 may contain a liquid cryogen 210 while the lower cryostat chamber contains a gaseous cryogen 275 as discussed in detail below in reference to FIG. 2. The cooled superconducting power cable system 100 may also comprise a gaseous cryogen recirculation system 135. The gaseous cryogen recirculation system 135 may circulate the gaseous cryogen 275 through the lower cryostat chamber 125 of the first cable termination 105, through the cable enclosure 130, and through the lower cryostat chamber 125 of the second cable termination 110.

Each of the upper cryostat chambers 120 may further comprise an upper bushing 115 that facilitates insertion of a conductor 225 into the upper cryostat chamber 120.

FIG. 2 illustrates various embodiments of a sectional view of each of the first and second cable terminations 105, 110. The upper cryostat chamber 120 may comprise a generally hollow interior cavity 205 at least partially filled with the liquid cryogen 210. An end of the upper cryostat chamber 120 positioned away from the lower cryostat chamber 125 may form an upper feed through 240 terminating at a flange 235. The upper bushing 115 may comprise an insulator body 230 that partially extends through the upper feed through 240 and into the upper cryostat chamber interior cavity 205. A portion of the upper bushing insulator body 230 may extend into the liquid cryogen 210. An upper bushing conductor 225 may extend through the insulator body 230 such that a first end 232 of the conductor 225 terminates outside the upper cryostat chamber 120 and the insulator body 230 such that electrical connections may be made with the first end 232. A second end 234 of the conductor 225 may terminate outside the insulator body 230 within the upper cryostat chamber interior cavity 205 and immersed in the liquid cryogen 210.

The purpose of the liquid cryogen 210 is at least twofold. First, the liquid cryogen 210 may intercept both the conductive heat leak from ambient and the Joule heating of the tipper bushing conductor 225. Second, the presence of the liquid cryogen 210 maintains the flange 297 of the lower cryostat chamber 125 at about 77 K, thus maintaining the dielectric strength of the gaseous cryogen 275 in contact with the flange 297. In various embodiments, the liquid cryogen 210 may be maintained at a temperature between about 60 K to about 80 K.

The lower cryostat chamber 125 may comprise a generally hollow interior cavity 270 at least partially filled with the gaseous cryogen 275. The lower cryostat chamber 125 may comprise a lower feed through 295 terminating at a flange 297. The upper cryostat chamber 120 and the lower cryostat chamber 125 may be coupled together such that the lower feed through 295 extends into the upper cryostat chamber 120 interior cavity 205. The lower feed through 295 effectively connects the upper cryostat chamber 120 interior cavity 205 with the lower cryostat chamber 125 interior cavity 270. In order to maintain the liquid cryogen 210 within the upper cryostat chamber 120 interior cavity 205 and the gaseous cryogen 275 within the lower cryostat chamber 125 interior cavity 270, a lower bushing 260 may extend through the lower feed through 295. The lower bushing 260 may form a seal at the flange 297 such that a portion of the lower bushing 260 is in contact with the liquid cryogen 210 and a portion of the lower bushing 260 is in contact with the gaseous cryogen 275. The seal formed by the lower bushing 260 may serve to retain the liquid cryogen 210 in the upper cryostat chamber 120 and retain the gaseous cryogen 275 in the lower cryostat chamber 125.

A lower bushing conductor 255 may extend through the lower bushing 260 such that a first end 257 of the lower bushing conductor 255 terminates within the upper cryostat chamber 120 in proximity to the second end 234 of the upper bushing conductor 225. The first end 257 of the lower bushing conductor 255 may be immersed in the liquid cryogen 210. The second end 234 of the upper bushing conductor 225 may be coupled to the first end 257 of the lower bushing conductor 255 by a flexible connector 250. The flexible connector 250 provides an electrical connection between the upper bushing conductor 225 and the lower bushing conductor 255. The flexible connector 250 may also reduce the mechanical stress at the connection due to different coefficients of expansion of the materials used for the upper bushing conductor 225 and the lower bushing conductor 255, or differences in the rate of expansion or contraction of the conductors 225, 255 as they change temperature. In addition, the flexible connector 250 may correct for any misalignment between the tipper bushing conductor 225 and the lower bushing conductor 255.

The lower bushing conductor 255 may have a second end 259 that extends beyond the lower bushing into the internal cavity 270 of the lower cryostat chamber 125 and into the gaseous cryogen 275. The superconducting power cable 280 may be coupled to the conductor by a clamp 265 or other coupling device.

A shape of the lower bushing 260 may be selected to minimize the electric field on an outer surface of the lower bushing 260. In various embodiments, the maximum electric field on the outer surface of the lower bushing 260 may be maintained below 50 V/mm, and the maximum electric field within the lower bushing 260 may be maintained below about 100 V/mm for 1,000V operating voltage. The temperature of the outer surface of the lower bushing 260 may be maintained at about 77 K to avoid any corona activity that could lead to degradation of the outer surface of the lower bushing 260 and eventually lead to electrical breakdown along the outer surface.

The lower cryostat chamber 125 may further comprise gaseous cryogen inlet/outlet ports 285 at either end. The gaseous cryogen inlet/outlet ports allow circulation of gaseous cryogen 275 through the gaseous cryogen recirculation system 140, as well as provide a passage for the superconducting power cable 280 to enter the lower cryostat chamber 125. According to various embodiments, a mass flow rate of the gaseous cryogen 275 through the gaseous cryogen recirculation system 140 may range from about 1 g/s to about 20 g/s, although both lower and higher mass flow rates are contemplated.

The upper bushing 115 and the lower bushing 260 may be constructed from one or more ceramic materials, one or more polymer materials, or any other material that provides the proper electrical resistance and temperature resistance to withstand exposure to the liquid cryogen 210 and the gaseous cryogen 275. In various embodiments, the upper bushing 115 and the lower bushing may be constructed from more than one material. For example, a portion of the lower bushing 260 exposed to the gaseous cryogen 275 may be constructed from one type of material, and a portion of the lower bushing exposed to the liquid cryogen 210 may be constructed from a second type of material.

The liquid cryogen 210 in the upper cryostat chamber 120 helps reduce heat influx into the lower cryostat chamber 125. Since the dielectric strength of a cryogen is generally stronger when the cryogen is in the liquid phase as opposed to the gaseous phase, a volume 220 (generally containing a gaseous form of the liquid cryogen 210) above the liquid level 215 of the liquid cryogen 210 should be kept free of any electric field. Internal field grading elements 245 in the upper bushing insulator body 230 may be made of conductive material and electrically grounded via connection to the flange 235. The internal field grading elements 245 may be long enough to extend below the liquid level 215 of the liquid cryogen 210. This reduces the electric field to a negligible level on an outer surface of the upper bushing insulator body 230 exposed to the vapor phase of the liquid cryogen 210 in the volume 220 above the liquid level 215 of the liquid cryogen 210, thereby maintaining volume 220 free of electric field.

In order to reduce the boil-off of the liquid cryogen, the heat influx from ambient, as well as ohmic losses in the upper bushing conductor 225, should be optimized [20]. This may require the upper bushing conductor 225 to have a small cross-section to minimize the heat influx from ambient and at the same time a large cross-section to reduce ohmic losses. The electric conductivity of the upper bushing conductor 225 may be substantially higher near the flexible connector 250 than near the upper feed through 240 because the temperature of the upper bushing conductor 225 is lower near the flexible connector 250. Therefore, various embodiments may comprise an upper bushing conductor 225 having a smaller cross-section near the flexible connector 250 than near the upper feed through 240. Through computer simulation and experimentation, it was determined that a transition region 227 where the cross-section of the upper bushing conductor 225 decreases should be positioned just above the flange 235.

The lower cryostat chamber 125 may be pressurized with high pressure gaseous cryogen 275 at cryogenic temperature. The temperature of the gaseous cryogen 275 may be maintained between about 50 K to about 70K. The arrangement illustrated by the various embodiments of FIG. 2 of the upper and lower cryostat chambers 120, 125 may allow a cold surface of the lower bushing 260 on the side of the gaseous cryogen 275 which helps maintain the dielectric properties of the cable terminations 105, 110. A single bushing spanning from ambient directly into the lower cryostat chamber 125 would not necessarily result in a cold surface of the lower bushing 260 in contact with the gaseous cryogen 275. The resulting reduced heat load to the lower cryogen chamber 125 due to the liquid cryogen 210 in the upper cryostat chamber 120 is an additional benefit.

In various embodiments, the gaseous cryogen 275 in the lower cryostat chamber 125 interior cavity 270 may be maintained at pressures in the range from about 50 psi to about 250 psi. The gas pressures may be maintained below 50 psi, although the ability of the gaseous cryogen 275 to provide adequate cooling may be limited. Gas pressures above 250 psi may also be used, although higher pressures may be limited by the pressure rating of various components of the system 100.

In various embodiments, either or both the upper cryostat chamber 120 and the lower cryostat chamber 125 may comprise a vacuum jacket 290. When the upper and lower cryostat chambers 120, 125 are coupled together, a continuous vacuum jacket 290 may be formed around the tipper and lower cryostat chambers 120, 125. The vacuum jacket may reduce heat influx from ambient into the cooled superconducting power cable system 100. The vacuum jacket may also minimize the thermal link between the upper cryostat chamber 120 and the lower cryostat chamber 125.

Any suitable liquid cryogen 210 and gaseous cryogen 275 may be used in the system 100. For example, the liquid cryogen 219 may be selected from liquid nitrogen, liquid argon, liquid neon, liquid oxygen, liquid methane, liquid hydrogen, liquid carbon dioxide, liquid nitrous oxide, liquid krypton, and mixtures thereof. Exemplary gaseous cryogens 275 may comprise a gaseous form of the liquid cryogens 210 listed previously.

Heat Load from the Terminations and the Cryostat

Heat leak from the cooled superconducting power cable system 100 was estimated from the design dimensions of the system 100. The two main sources of heat influx into the system 100 are (1) heat conduction through the walls of the first and second cable terminations 105, 110 and the cable enclosure 130, and (2) heat conduction and Joule heating of the upper and lower bushing conductors 225, 255 and electric connection points.

The influx through the walls the first and second cable terminations 105, 110 and the cable enclosure 130, {dot over (Q)}_(termination), was estimated by apparent thermal conductivity based on values found in the literature [17, 18] for multi-layer insulation of similar specifications as those used herein. Assuming a residual gas pressure of 0.1 Pa (cold vacuum pressure) for the first and second cable terminations 105, 110, the apparent thermal conductivity is approximately k_(term)=1.7·10⁻³ W/(m·K). The conductive influx through the upper and lower bushing conductors 225, 255 was estimated using Fourier's law. Additional heat influx is from the cable enclosure 130, {dot over (Q)}_(cryostat), and two bayonet connectors (not shown in the figures), {dot over (Q)}_(connector), between the first and second cable terminations 105, 110 and the cable enclosure 130. Values of heat leak through the cable enclosure 130 and the upper and lower bushing conductors 225, 255 were based on a previous study [8]. At no-load condition (i.e., no Joule heating), the resulting heat influx is

${\overset{.}{Q}}_{tot} = {{{2{\overset{.}{Q}}_{termination}} + {\overset{.}{Q}}_{cryostat} + {2{\overset{.}{Q}}_{connector}}} = {{{2\left( {{k_{term}\frac{A_{term}\Delta\; T_{term}}{d_{term}}} + {k_{Cu}\frac{A_{Cu}\Delta\; T_{Cu}}{d_{Cu}}}} \right)} + {\overset{.}{Q}}_{cryostat} + {2{\overset{.}{Q}}_{connector}}} = {{{2\left( {{20W} + {19\; W}} \right)} + {30\; W} + {20\; W}} = {128\; W}}}}$ with input variables summarized in Table 1. There will be additional heat loads by Joule heating from contact resistances between various components, such as current leads, clamshells, connectors, and cable termination blocks, when current is applied.

TABLE 1 Summary of parameters to estimate heat flux from different sources. Copper Parameter Termination Current Lead (Apparent) thermal conductivity k 1.7 × 10⁻³ 647 [W/(m · K)] Surface area (cross section) A [m²] 1.25 5.1 × 10⁻⁴ Thickness (length) [mm] 25.4 477 Temperature difference ΔT [K] 250  27

Experimental Approach

To measure the heat leak into the superconducting DC cable system, experiments were conducted on the first and second cable terminations 105, 110 and cable enclosure 130 without the actual superconducting power cable 280 in the system. Temperature profile across the assembly was measured as a function of the mass flow rate of gaseous cryogen 275.

Three temperature sensors were positioned in the lower cryostat chamber 125 interior cavity 270 along the centerline from the gaseous cryogen inlet/outlet ports 285. Two of the sensors were placed in proximity to the inlet/outlet ports 285 and the third sensor was positioned in proximity of the clamp 265 to match the position of the superconducting power cable 280 during actual operation. An additional temperature sensor was positioned in proximity to the bottom of the lower bushing 260. The locations of the temperature sensors were identical in both first and second cable terminations 105, 110.

Two cable enclosures 130 (cryostats) were used in the experiments. One of the cable enclosures 130 was a 1 m long custom made rigid vacuum jacketed cryostat with super insulation. The second cable enclosure 130 was a 30 m long flexible cryostat manufactured by Nexans with inner and outer diameters of 39 and 66 mm, respectively [“CRYOFLEX® Transfer Lines for Liquid Gases”, Nexans, September 2003]. Both cable enclosures 130 were connected to the first and second cable terminations 105, 110 with bayonet-type connections.

The gaseous cryogen recirculation system 135, using helium gas, operated at pressures up to 250 psi and provided mass flow rates up to 8 g/s. In general, with this mass flow rate, the gaseous cryogen recirculation system 135 can provide up to 400 W cooling power for a temperature rise of 10 K across the system 100. The temperature at the inlet to the gaseous recirculation system 135 was in the range of 40-75 K, depending on the cable enclosure 130 used, operating pressure, and mass flow rate.

To assess the benefit of liquid nitrogen in the upper cryostat chamber 120 in reducing the heat load to the lower cryostat chamber 125, experiments were performed with and without liquid nitrogen in the upper cryostat chamber 120. When liquid nitrogen was not used, the upper cryostat chamber 120 was vacuumized down to less than 10⁻⁶ bar in order to provide thermal insulation.

Results and Discussion

The first and second cable terminations 105, 110 were first connected with cable enclosure 130 (i.e., the 1 m long enclosure) and the system 100 was evacuated followed by flushing with helium gas. The process of evacuating and flushing with helium gas was repeated three times before activating the cooling system. A similar procedure was followed for the experiments with the 30 m long cable enclosure 130. Experiments were conducted with and without liquid nitrogen in the upper cryostat chamber 120 to assess the savings on heat load into the lower cryostat chamber 125 reduced by the liquid nitrogen.

Experiments without Liquid Nitrogen in the Current Lead Chamber

FIG. 3 shows a graph of the temperatures at the first and second cable terminations 105, 110 of system 100 at several mass flow rates without liquid nitrogen in the upper cryostat chamber 120. As expected, the temperature rise across the cable enclosure 130 decreased with the increase in mass flow rate initially and leveled off at around 4 g/s.

FIG. 4 shows a graph of the variation in the temperature rise across the entirety of the system 100, including first and second cable terminations 105, 110 and cable enclosure 130 as the mass flow rate of helium gas is increased. For the 1 m long cable enclosure 130, the temperature rise dropped from 20 K for 1 g/s to 3.5 K for 6 g/s. Similarly, for the 30 m long cable enclosure 130, the temperature rise dropped from 16 K for 1.7 g/s to 5.2 K for 7 g/s.

For each mass flow rate, the temperatures at all the locations were recorded after the system 100 reached a steady state indicated by stable temperature for 20 minutes. Approximate heat leak from each of first and second cable terminations 105, 110 and the two cable enclosures 130 could be estimated from the experimental steady state temperature rise across each of the components, mass flow rate, and the heat capacity of helium gas (5 J/g-K). FIG. 5 shows a graph of the calculated heat load from each of the components for two sets of the experiments, one with the 1 m cable enclosure 130 and the other with 30 m cable enclosure 130.

As seen in FIG. 5, the calculated heat loads for both first cable termination 105 and second cable termination 110 from the temperature rise data of each flow rate and cable enclosure 130 size are similar in magnitude and in the range between 35 and 55 W. The calculations are based on the assumptions that there is complete thermal mixing of the gas and no stratification in first and second cable terminations 105, 110. These assumptions may not be valid in first and second cable terminations 105, 110 where the gaseous cryogen inlet/outlet 285 and the cable enclosure 130 are significantly narrower at 39 mm in inner diameter compared to the bulk of the first and second cable terminations, where the inner diameter is 254 mm. This effect may vary with the mass flow rate and the operating pressure.

Heat leak from the cable enclosure 130 was estimated from the temperatures at the gaseous cryogen outlet 285 of the first cable termination 105 and the gaseous cryogen inlet of the second cable termination 110. The cable enclosure 130 was coupled to the first and second cable terminations 105, 110 via bayonet connection and a Teflon O-ring. The heat leak calculated for the 1 m cable enclosure 130 was less than 1 W/m. The heat leak from the 30 m cable enclosure 130 was in the range of 2.5-3 W/m. This value is slightly higher than the expected 1-2 W/m based on the manufacturer data and previous reports of such studies [18, 19]. The cable enclosure 130 has been used for two years in many experiments with high pressure helium gas. The higher heat load than expected suggests that there may be slight deterioration of vacuum quality. The calculated heat load increases from 2.5 W/m to 3 W/m with the increase in the mass flow rate due to promoting convective heat transfer on the wall of the cable enclosure 130 because of higher flow velocity.

Experiments with Liquid Nitrogen in the Current Lead Chamber

To quantify the advantage of using liquid nitrogen in the upper cryostat chamber 105, experiments were repeated after filling the upper cryostat chamber 120 of both the first and second cable terminations 105, 110 with liquid nitrogen. Temperature rises in both the first and second cable terminations 105, 110 and the cable enclosure 130 were measured at various mass flow rates similar to the set of experiments without the liquid nitrogen described previously. As expected, the temperature rise is smaller confirming the heat load through the upper cryostat chamber 120 to first and second cable terminations 105, 110 is less.

FIG. 6 shows a graph of the calculated heat loads from first and second cable terminations 105, 110 and cable enclosure 130 for several flow rates for the experiments with liquid nitrogen in the upper cryostat chamber 120. It can be seen that the heat load from the cable enclosure 130 is similar to the set of experiments without the liquid nitrogen in the upper cryostat chamber 120 for both the 1 m and 30 m cable enclosure 130 cases. The heat load from the first and second cable terminations 105, 110 is around 20 W, approximately 30-40 percent lower than for the set of experiments without the liquid nitrogen in the upper cryostat chamber.

The heat load from the first and second cable terminations 105, 110 calculated from the temperature rise data represents the heat load without any current through upper bushing 115 and lower bushing 260. The results of the experiments suggest that each of the first and second cable terminations 105, 100 required a cooling power of 20-40 W and the cable enclosure 130 required approximately 2 W/m making the total required cooling power to remove the heat leak into system 100 of 100-140 W in order to operate the power cables 280 at 40-50 K. Additional cooling power is needed to compensate for the conductive and Joule heating of the conductors 225, 255 under operating current.

Thermal studies on the cable terminations 105, 110 and a 30 m cable enclosure 130 meant for testing a superconducting DC monopole cable 280 cooled with helium gas circulation provided a thermal map of the system 100 without the actual superconducting cable 280. The total heat leak into the system 100, estimated from the measurements of temperature profile across the test system 100, was approximately 100 W when liquid nitrogen is used in the upper cryostat chamber 120 of the cable terminations 105, 110. Absence of the liquid nitrogen increased the total heat load to 140 W. The temperature rise across the cable terminations 105, 110 and cable enclosure 130 assembly was 4-8 K, depending on the mass flow rate of helium circulation. The temperature profile across the test system 100 was used to estimate the heat load from different components of the system 100,

GLOSSARY OF CLAIM TERMS

Bushing: An insulator positioned in an opening to facilitate positioning on a conductor within the opening and to electrically isolate the conductor.

Conductor: A conductive material, for example copper, aluminum or brass, used to make an electrical connection.

Cryogen: A substance used to produce low temperatures (typically less than about −150° C.).

Cryostat: A device used to maintain cryogenic temperatures of samples or devices within the device.

Field grading element: An electrically conductive structure that reduces an electric field on the surface of an object.

Power cable: A cable used to conduct electric energy. In certain situations, the power cable may be a superconducting power cable.

Power cable enclosure: An enclosure surrounding a power cable that also allows a gas to pass through the enclosure.

Power cable termination: a device for connecting a power cable to a bus, cables, or other conductor.

Superconductor or superconducting: A material for which electrical resistance drops to zero below the critical temperature of the material.

REFERENCES

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All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

It will thus be seen that the objects set forth above, and those made apparent from the foregoing disclosure, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing disclosure or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween. 

What is claimed is:
 1. An apparatus for maintaining dielectric integrity and thermal management of an electric connection, comprising: a power cable termination comprising an upper cryostat chamber and a lower cryostat chamber; an interior cavity within the upper cryostat chamber at least partially filled with a liquid cryogen; an interior cavity within the lower cryostat chamber at least partially filled with a gaseous cryogen; an upper bushing extending into the upper cryostat chamber; a lower bushing extending into the lower cryostat chamber; an upper bushing conductor passing through the upper bushing and into the interior cavity of the upper cryostat chamber such that at least a portion of the upper bushing conductor is outside the cable termination and at least a portion of the upper bushing conductor is immersed in the liquid cryogen; a lower bushing conductor passing through the lower bushing and into the lower cryostat chamber such that at least a portion of the lower bushing conductor is positioned within the gaseous cryogen and at least a portion of the lower bushing conductor is positioned within the liquid cryogen a power cable extending into the interior cavity of the lower cryostat chamber, the power cable electrically coupled to the lower bushing conductor.
 2. The apparatus of claim 1, wherein the upper bushing conductor and the lower bushing conductor are coupled by a flexible connector, the flexible connector configured to reduce the mechanical stress at the connection due to different coefficients of expansion of the first and second sections or differences in the rate of expansion or contraction of the upper bushing conductor and the lower bushing conductor.
 3. The apparatus of claim 1, wherein the upper bushing comprises a field grading element within the upper bushing and electrically coupled to the upper cryostat chamber, the field grading element formed of a conductive material and grounded via coupling to the upper cryostat chamber, the field grading element extending into a liquid level of the liquid cryogen to reduce electric field to a negligible level on a surface of the upper bushing that is exposed to a vapor phase of the cryogenic liquid.
 4. The apparatus of claim 1, wherein the lower bushing is in contact with both the liquid cryogen and the gaseous cryogen.
 5. The apparatus of claim 1, herein a portion of the upper bushing conductor outside the cable termination has a first cross-sectional diameter and a portion of the upper bushing conductor within the interior cavity of the upper cryostat chamber has a second cross-sectional diameter, the first diameter being greater than the second diameter.
 6. The apparatus of claim 5, wherein a transition region where the cross-sectional diameter of the upper bushing conductor reduces from the first diameter to the second diameter is located outside the cable termination.
 7. The apparatus of claim 1, further comprising a cable enclosure coupled to the lower cryostat chamber, the power cable positioned within the cable enclosure.
 8. The apparatus of claim 7, further comprising a gaseous cryogen recirculation system to move gaseous cryogen through the lower cryostat chamber and through the cable enclosure. 